Heat transfer devices, specifically heat exchangers, find all kinds of application from industrial operations to medical ventilation techniques. Simple heat exchangers transfer heat from an outgoing hot fluid to an incoming cool fluid. More recently, heat exchangers have utilized the regenerator principle.
The regenerator principle involves heat recovery when a fluid (referred to as a "shuttle fluid") is reciprocally exchanged between two reservoirs of different temperature, i.e., alternating flow by a hotter or colder fluid with some mechanism for effecting this reciprocating fluid flow through the system. The two-part regenerator cycle consists of flow of the fluid from the cold to the hot reservoir through a regenerator bed (or matrix) of porous heat transfer material, followed by flow of the fluid from the hot to the cold reservoir through the bed.
Where the heat capacity of the bed is very large compared to the heat capacity of the shuttle fluid, a temperature profile is established in the regenerator bed. The shuttle fluid is the total fluid mass that flows in one direction prior to reversal. After many reciprocating flows, the bed material establishes a temperature profile that increases from the side at which the cold fluid enters to the side at which the hot fluid enters. During the flow from cold to hot, the fluid enters at temperature T.sub.C, the temperature of the cold heat exchanger. It is warmed by the bed as it passes through the bed, and leaves the bed at a temperature below T.sub.H, the temperature of the hot exchanger. During flow from hot to cold, the fluid enters the bed at temperature T.sub.H. It is cooled by the bed as it passes through and leaves the bed at a temperature above T.sub.C. This difference in temperature of the fluid from entrance to exit from the bed, .DELTA.T, causes heat flow from the hot to cold reservoir. At worst, it is T.sub.H -T.sub.C, if there were no regenerator present. The ratio of .DELTA.T to (T.sub.H -T.sub.C) is referred to as the regenerator ineffectiveness. Over the cycle, the bed receives no net heat. It acts as an intermediate heat reservoir, absorbing heat from the warm gas and rejecting it to the cool gas.
Regenerative air-to-air heat exchangers are known; see, e.g., U.S. Pat. No. 4,875,520 issued to Steele et al. In such regenerators, it is important for the bed to have high heat capacity.
In other types of heat exchangers, two fluid streams are separated by the bed and flow continuously; here the heat capacity of the bed is irrelevant. See, for example, U.S. Pat. No. 4,858,685 issued to Szucs et al. It is important, however, in these cases, for the bed to have excellent thermal conductivity as well as high moisture transfer (when moisture retention is desirable).
Other known heat exchangers include U.S. Pat. No. 4,744,414 issued to Schon (continuous heat exchanger); U.S. Pat. No. 4,574,872 issued Yano et al. U.S. Pat. No. 4,574,872 (regenerative rotary type total heat exchanger); U.S. Pat. No. 4,577,678 issued to Franenfeld et al. (plastic storage material for heat transfer).
Another type of heat exchanger is an evaporative cooler which is disclosed in, e.g., U.S. Pat. No. Reissue 26,560 issued to Meredith. In the Meredith device, there is continuous flow of the liquid and gas, the two fluids. Air is forced through a matrix where the evaporating water cools and the air is, in turn, cooled in the matrix. A desirable matrix has high moisture absorption; thermal conduction and heat capacity are irrelevant. Meredith discloses a matrix using sheeting, fabric or filaments placed under tension. The liquid adheres to the fabric (matrix) as it flows downward. The fabric (matrix) performs no thermal function, and the liquid and gas are in direct contact. No heat is transferred to or through the fabric.
Moreover, a device such as that disclosed by Meredith is quite complex and uses plumbing and sprayers, for the water must flow down the fabric sheets or filaments while air flows between them. The layers must have separation significantly greater than the size of a water drop, "3/8 inch, more or less." The flow of air is turbulent, and Meredith describes the problem of "flutter" caused by angular rotation of a long, narrow strip of fabric in a fast moving turbulent air stream.
A specialized application of heat exchangers is in pulmonary medical devices for artificial ventilation. When human patients are on anesthesia machines during surgery or respirators during long-term pulmonary care, a ventilator is used to perform breathing for the patient. The ventilator forces air into the lungs via an endotracheal tube inserted into the trachea, then allows the lungs to exhale due to their compliance. This artificial ventilation is distinguished from normal breathing because the patient's nose is bypassed for breathing. As a consequence, the heating and moisturizing function of the nose is circumvented. It is well known that the cold, dry air impinging on a patient's respiratory airways, i.e., the trachea and the bronchial tubes, damages the lining of the airways over time.
Deep in the lungs, the measured state of air is 37.degree. C. and 100% relative humidity, corresponding to an absolute moisture content of 44 mg of water per liter of air. Dry medical gases used for patient ventilation have temperatures at about 20.degree. C. and a relative humidity (RH) of 0%, corresponding to an absolute moisture content of 0 mg/L of air. A perfect regenerative heat exchanger which transmits sensible heat only will reduce the temperature of the exhaled air to 20.degree. C. Sensible heat is that portion of total heat content of air which can be sensed by a thermometer. Saturated air at 20.degree. C. corresponds to an absolute moisture content of about 16 mg/L of air. Thus, in reducing the temperature of air to 20.degree. C., 44mg/L minus 16 mg/L of moisture must be removed from the exhaled air. This moisture removal is achieved by condensing a thin film of moisture on the surface of the heat exchanger bed. In the case of exhalation, the patient will lose 16 mg of water per L of air exhaled and 28 mg of water per L of air (44 minus 16) will be condensed on the surface of the bed. On inhalation, the dry air will evaporate this thin film of water and the bed will warm the air to 37.degree. C. Thus, the inhaled air will contain 28 mg absolute moisture per L of air.
Active heaters and moisturizers, typically, are used to provide warm moist air to the patient. Heated water humidifiers are used to insert moisture and heat into the incoming air flowing into the endotracheal tube. These devices and methods pose many problems. Electric heaters are used to evaporate the water, which must be sterile distilled water; the devices thus require monitoring and refilling. The devices also represent a potential shock and burn hazard to the patient and to those assisting in the patient's care. In addition to these problems, the exhaled air frown the patient has a dew point well above the temperature of the tubes which receive the exhaled air. Thus, condensation occurs in these tubes. Water traps are typically used to collect the condensation. The water traps also must be monitored and emptied on a regular basis. This condensation water can also be a biohazard. Thus, the active heat/moisture devices are expensive to operate and maintain, and pose significant patient risk.
Passive heat and moisture exchangers (HMEs) are also used. Like the active heaters and moisturizers, the primary function of an HME is to prevent patient moisture loss from the linings of the trachea and lungs.
For patient use in medical artificial ventilation, the HME devices are typically enclosed in a housing and are positioned in-line between the endotracheal tube and the Y-connector line to the ventilator, i.e., the patient breathes through the HME.
As explained hereinbefore, a heat and moisture exchanger should be able to absorb and desorb as much as 44 mg of water per L of air. Theoretically, an HME can return almost all of the exhaled moisture from the breath. Yet, even the best, currently available, return only about two-thirds of the maximum 44 mg of water per L. Return of the maximum amount of moisture is possible if hygroscopic materials are incorporated into the exchanger. With a hygroscopic material, water can be absorbed and desorbed at moisture levels below 100% relative humidity.
HMEs have been used for respiratory air heating both nonmedically and medically. A recent example of nonmedical use is SOUTHWIND.TM. RESPIRATOR, available from CenTex, Carbondale, Pa., Part No. WW-10. Nonmedical humidifying and air warming masks for a variety of bed materials include, for example, U.S. Pat. No. 3,814,094 to DeAngelis et al. (use of stacks of aluminum screening); U.S. Pat. No. 4,294,242 to Cowans (stainless steel screening); U.S. Pat. No. 5,010,594 to Suzuki et al. and U.S. Pat. No. 5,007,114 to Numano (bast paper fibers); U.S. Pat. No. 4,325,365 to Barbuto (spaced curved leaf members of aluminum or copper); U.S. Pat. No. 3,326,214 to McCoy (convolutely wound aluminum foil or pleated aluminum foil); U.S. Pat. No. 4,620,537 to Brown (concentric shells of hygroscopic cellulose and felt); U.S. Pat. No. 4,825,863 to Dittmar et al. (electrical heaters and humidifier cartridges); U.S. Pat. No. 2,610,038 to Phillips (spiral wound sheet material forming inspiratory channels with absorbent material attached to or incorporated in the walls of the channel); U.S. Pat. No. 3,333,585 to Barghini et al. (porus hydrophobic fabrics).
Use of hygroscopic materials as inserts for a moisture and heat exchange device for breathing devices are also known; see, e.g., U.S. Pat. No. 4,771,770 to Artemenko et al. (alternating hydrophobic and hydrophilic insert washers); U.S. Pat. No. 3,747,598 issued to Cowans (hygroscopic activated molecular sieve material); U.S. Pat. No. 3,099,987 issued to R. G. Bartlett Jr. (silica gel); U.S. Pat. No. 3,920,009 issued to Olsen (tracheostomatic bandage HME). Some HMEs rely on condensation and evaporation; see, e.g., U.S. Pat. No. 3,920,009 issued to Olsen.
Earlier medical HMEs consisted of stacked aluminum screens. Later versions of HMEs have employed a fiber mass with a desiccant such as lithium bromide or lithium chloride (an example of such an HME is the ARC.TM. device made by ARC Medical, Inc. Pharma Systems AB, Sweden). Other HME designs utilize laminar flow structures or quasi-laminar flow structures. In one device, a long strip of corrugated paper, treated with desiccant, is wrapped in a spiral (See, for example, Humid-Vent.TM. 2 Port, Gibeck-Respiration AB, Uplands Vasby, Sweden). In another device, two long strips of treated paper, each corrugated at about a 45 degree angle with respect to the length of the strip, are placed together. This combined strip is then spiral wrapped. (See, for example, HYGROBAC.TM. made by DAR, Mirandola, Italy).
HMEs are also known that utilize pleated or folded sheet material (see, U.S. Pat. No. 5,035,236 to Kanegaonkar) along with hygroscopic inserts or bars are also used. (See U.S. Pat. No. 5,320,096 to Hans). Other HMEs for use with medical ventilators include the use of vapor permeable fiber tubes with or without warm water circulated into the housing space around the tubes; see, e.g., U.S. Pat. No. 4,355,636 to Oetjen et al.; UK Pat. No. Application GB 2,082,921 to Benthin; U.S. Pat. No. 4,327,717 to Oetjen et al. Metered tempered sterilized water can also be supplied to the HME; see, e.g., U.S. Pat. No. 5,383,447 to Lang.
There has been considerable effort in developing hygroscopic coatings or treatments for use in HMEs. With current HMEs, the preferred approach is to use water soluble salts such as lithium chloride (LiCl) or calcium chloride (CaCl.sub.2) as the hygroscopic material. These materials readily absorb and desorb water vapor, forming a thin water-salt solution on their surface. Since these materials are water soluble, patient moisture may leach the salts, reducing their effectiveness over time. Some liquids such as glycerin, and ethylene glycol readily absorb and desorb moisture and can be used as a hygroscopic material. In order to use these liquids, an HME bed must contain a solid hygroscopic absorbent, such as a paper fiber. As with water soluble salts, the hygroscopic material may leach over time, and the liquids which exhibit a nonzero vapor pressure will evaporate with time. U.S. Pat. No. 5,320,096 further discloses treating strips of heat and moisture exchanging material with LiCl, CaCl.sub.2, polyacrylic acid, polyvinyl pyrrolidone, polyvinyl alcohol, or other hydrophilic polymers, glycol or glycerin. However, the hygroscopic treatments are water soluble and can migrate or leach with time.
Partitioning the hygroscopic material with grids of hydrophobic material has been used to prevent flow of the hygroscopic salt; see, U.S. Pat. No. 4,594,860 to Coeliner et al. (a moisture transfer wheel).
Solid crystalline or amorphous inorganic materials are frequently used as desiccants in heat recovery ventilation applications. They have long life and provide excellent moisture return performance. For example, molecular sieves such as silica gel (amorphous silicon dioxide), activated carbon, and zeolites (crystalline sodium aluminosilicate) are often used. The particles are held in place in various ways; see, e.g., U.S. Pat. No. 3,099,987 (screens); U.S. Pat. No. 4,875,520 (bonding technique).
Virtually all presently available HMEs are known to be only partially effective. They do not retain sufficient moisture to prevent drying of the patient's airways. For patient's on long term pulmonary care, the drying of the airways which occurs with presently available HMEs thickens mucous secretions over time to the point where blockage occurs. This is a life threatening situation, often leading to pneumonia. After several days, HMEs must be replaced with active heat and moisturization. There is a great need for a highly effective HME which can provide sufficient moisture retention to the patient to prevent drying of mucous secretions.
It is noted, in addition to the need for HME with high moisture retention, there is a need for a device with low resistance to flow or low pressure drop and low dead volume. By "dead volume" is meant the total internal volume of the HME which is air. At the end of the out-breath, this volume is filled with stale air from the patient's exhalation which the patient breathes back in again on the next in-breath. It is important to keep the dead volume small compared to the total volume of inhaled air so that the patient receives air sufficiently rich in oxygen and low in carbon dioxide. Resistance to flow and large dead volume impede weaning of the patient from the ventilator and subsequent recovery.
Thus, notwithstanding the many known problems with current mechanical systems and the practical design problems of alternative systems, the art has not adequately responded to date with a heat exchanger that utilizes regeneration and allows a high heat transfer between the fluid and the heat exchanger for near room temperature regenerator applications. Further, the art has not produced an HME that can provide sufficient moisture retention to prevent drying of a patient's airways and that has low resistance to flow or low pressure drop and low dead volume. Nor has the prior art produced regenerative heat exchangers utilizing a hygroscopic elastomeric matrix which can directly absorb and desorb a substantial amount of water vapor thereby increasing the heat capacity of the matrix.